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View Full Version : Attention geologist dopers - pyritic shale


madrabbitwoman
04-22-2009, 09:08 PM
Hi all,
I am a very amateur geologist. Recently while going though a box of rocks I found a baggie containing a laminar black rock covered in white/grey powdery stuff labeled pyritic shale. I know what shale is and how it is formed but don't know so much about pyritic shale. It smells sulfury and I am not quite game to touch it.
Am I correct in an assumption that the white powdery stuff is the result of some sort of chemical reaction? Will whole lot of this eventually end up as nothing more than grey powder? Is it safe to touch? Is there some way to store it so that it doesn't disintegrate?

Cheers

Squink
04-22-2009, 09:40 PM
My fool's gold is falling apart! Why? (http://boards.straightdope.com/sdmb/showthread.php?t=512487&highlight=pyrite)

Polycarp
04-22-2009, 10:04 PM
While pebble-size and larger pyrite is "fool's gold", there's no reason FeS2 cannot exist in much finer conditions -- except for its tendency to oxidize. When clays are laid down in anaerobic environments, they could and often did include some FeS2, producing pyritic shale when compressed to rock. And when exposed to air, the FeS2 will oxidize, producing iron oxide and sulfur dioxide -- rotten egg smell -- along with other iron-sulfur-oxide compounds. From a cursory Google search of articles mentioning pyritic shale, there appear to have been a lot of formations laid down in the later Proterozoic (end of the pre-Cambrian) and the first four periods of the Paleozoic -- say from 1,000 to 400 million years ago.

madrabbitwoman
04-22-2009, 11:57 PM
While pebble-size and larger pyrite is "fool's gold", there's no reason FeS2 cannot exist in much finer conditions -- except for its tendency to oxidize. When clays are laid down in anaerobic environments, they could and often did include some FeS2, producing pyritic shale when compressed to rock. And when exposed to air, the FeS2 will oxidize, producing iron oxide and sulfur dioxide -- rotten egg smell -- along with other iron-sulfur-oxide compounds. From a cursory Google search of articles mentioning pyritic shale, there appear to have been a lot of formations laid down in the later Proterozoic (end of the pre-Cambrian) and the first four periods of the Paleozoic -- say from 1,000 to 400 million years ago.

Do you think that storing it in water or oil may stop it oxidising?